Chapman, S. C.Smail, I.Ivison, R. J.Helou, G.Dale, D. A.Lagache, G.2014-03-122014-03-122002-07Chapman, S. C., I. Smail, R. J. Ivison, G. Helou, et al. 2002. "Optically faint counterparts to the ISO-FIRBACK 170 micron population: the discovery of cold, luminous galaxies at high redshift." The Astrophysical Journal 573(1): 66-740004-637Xhttp://dx.doi.org/10.1086/340552http://hdl.handle.net/10222/45523We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170micron-selected sources from the ISO-FIRBACK survey which have faint counterparts in the optical, and r-K~5. Both sources were expected to lie at z>1 based on their far-infrared, submillimeter and radio fluxes, assuming a similar spectral energy distribution to the local ultra-luminous infrared galaxy (ULIRG) Arp220. However, our spectroscopy indicates that the redshifts of these galaxies are z<1: z=0.91 for FN1-64 and z=0.45 for FN1-40. While the bolometric luminosities of both galaxies are similar to Arp220, it appears that the dust emission in these systems has a characteristic temperature of 30K much cooler than the ~50K seen in Arp220. Neither optical spectrum shows evidence of AGN activity. If these galaxies are characteristic of the optically faint FIRBACK population, then evolutionary models of the far-infrared background must include a substantial population of cold, luminous galaxies. These galaxies provide an important intermediate comparison between the local luminous IR galaxies, and the high redshift submillimeter-selected galaxies, for which there is very little information available.article573166